Efficient planning by effective resource reasoning

Planning consists of an action selection phase where actions are selected and ordered to reach the desired goals, and a resource allocation phase where enough resources are assigned to ensure the successful execution of the chosen actions. In most real-world problems, these two phases are loosely coupled. On the other hand, most existing planners do not exploit this loose-coupling and perform both action selection and resource assignment employing the same algorithm. The current work shows that the above strategy severely curtails the scale-up potential of existing planners, including such recent ones as Graphplan and Blackbox. In response, a novel planning framework was developed in which resource allocation is de-coupled from planning and is handled in a separate “scheduling” phase. Implementing this framework raises several interesting issues regarding the role of resources in planning, the interactions between the planning and scheduling phases and the choices in selecting the methods for the two phases. During planning, resource constraints are ignored and an abstract plan is produced that can correctly achieve the goals but for the resource constraints. Next, based on the actual resource availability, the abstract plan is allocated resources to produce an executable plan. This work introduces a procedural method for inexpensive (backtrack-free) scheduling and a declarative method for posing the scheduling problem with all its complexity as a Constraint Satisfaction Problem. The new approach not only preserves both the correctness as well as the quality (measured in length) of the plan but also improves efficiency. This is implemented on top of Graphplan and shows impressive empirical results. Beyond the context of planner efficiency, the current work can be viewed as an important step towards integrating planning with real world problem solving. Specifically, when a plan fails to achieve its intended purpose during plan execution, it does not imply that the causal structure of the failed plan was incorrect but that some allocated resources were found to be unavailable. The benefit of the current approach is that it provides a framework to undertake only necessary resource re-allocation and not complete re-planning.

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